اثر تغییر اقلیم بر بارش برف و پیش بینی آن در شهرستان همدان

The effect of climate change on snowfall and its prediction in Hamadan



Abstract

This research aimed at investigating and predicting snowing changes in Hamedan region using MODIS satellite data and images, harmonic analysis and micro-scaled atmospheric data. The study used Hamadan station data, satellite images and the NDSI index during the 30 years (1992 to 2022), analyzed the trend of snowfall changes, and modeled and predicted for the next 20 years. The analysis of satellite images and the NDSI index showed that the snow cover in Hamadan city has significantly decreased. This amount, compared to the area of the city, has decreased from about 20% of the snow cover to about 7% over the decade. Also, the variance value in the first harmonic coefficient during the statistical period is about 75%, which refers to seventy-five percent of snowfall in the region. The variance of the second harmonic coefficient is 20 percent, which implies the effect of Alvand Mountain and the height of the region on snowfall. In recent years, this amount has reached 27 percent, indicating a decrease in large-scale synoptic systems' contribution. It also shows climate changes and warmer winters. The maximum snowfall in the first harmonic is 16.2 centimeters, which has decreased to 13.2 centimeters in recent years. In recent years, the phase angle or the time of maximum snowfall has changed from mid-fall to early winter. The LARS micro-scale model was used to predict and scale the data of the atmospheric general circulation model (HadGEM2-ES) and the Coupled Model Inter-comparison Project (CMIP5). The results showed that Hamadan has been facing a decrease in snowfall and warmer winters in recent years.



Keywords: Changes in the snowfall regime, NDSI, harmonic analysis, modeling, Hamadan























































The effect of climate change on snowfall and its prediction in Hamadan city





Introduction

Investigating temporal and spatial changes in snowfall is necessary for monitoring and planning water resources. The melting of the snow provides part of the water needed for agriculture. In addition to agriculture, snow becomes more critical in providing drinking water when drought prevails in the region, especially in arid and semi-arid regions. Snow cover affects climate significantly through regional radiant energy and atmospheric and thermal circulation. Any climate changes may alter the amount and distribution of snow and will have long-term environmental and economic consequences. In high mountains, snow is the essential type of precipitation, whose increase and decrease affects the activities of other atmospheric phenomena, agriculture and even tourism. Besides, snow feeds river flows in spring and summer. The high light reflection of snow reflects many solar rays to the earth and causes gradual warming in spring. All in all, the amount and duration of snow are critical to ecological, economic and social systems, especially in the mountainous areas of Zagros.



Materials (Data) and methods

In this study, the snowfall data from Hamedan Airport synoptic station has been used. The 30 years (1992-2020) were chosen for the study time. Harmonic analysis was used to investigate changes in the snowfall regime in Hamadan city. To analyze more precisely, components such as trend and changeability were extracted from the data. The trend was determined in the linear regression function (assumed linear) and the least square method, which was significant at the confidence level. Mann-Kendall non-parametric test detected any possible trend in the statistical series of snowfall and checked the significance level of existing trends. In short-term trends, change of position or the starting point of the trend in the snowfall series were also investigated by the graphs of time series with (t) u and (t) ú values. Also, MODIS snow cover data with a resolution of 0.05 degrees was utilized to investigate changes in snowfall in the studied area.



Results and Discussion

The analysis of data shows that the height of snow in Hamedan station has decreased by 9 centimeters. The rapid increase in temperature in winter and the decrease in snowfall indicates the effects of climate change and global warming in Hamedan. According to Kendall coefficients, ui and ui' graphs present snow height in the winter season at Hamadan station. These graphs show a significant trend in the time series of snow height in the winter season at Hamadan station. In addition, sudden changes in the trend have been observed since 2017. Therefore, the reduction of snowfall and the relatively warm temperature in the winter season in Hamedan city is evident. Based on the predicted data in the LARS model, the minimum temperature will increase by 2 °C and the maximum temperature by 2.5 °C from 2020 to 2040.

Also, examining the changes in the amount of snowfall in Hamedan city shows that the trend of snowfall in Hamedan city is decreasing in the three season’s winter, fall and spring. Also, the coefficient of the snowfall trend changes in the winter is 38.8 compared to the baseline year. The coefficients of these trends are 37.2 in the fall and 36.00 in the spring. These changes result from climate changes on the earth and changes in climate elements. The decrease in snowfall has had destructive consequences on the water cycle in Hamedan city, so the accumulation of snow decreased compared to the base year, and the headwater sources have faced the risk of drying up.



Conclusion

The results indicate the difference in harmonic coefficients, the maximum amount of snowfall and the time of snowing in recent years. The increase in the second harmonic coefficient indicates the decrease in snowfall due to changes in macro-scale systems or the main challenge of the earth's ecosystem in recent years. The changes in time or phase angle also suggest that the snow is almost stopped in the fall, and the snowing time is delayed toward the winter. In other words, the fall season has become warmer in the region. Based on the height, the maximum amount of snowfall is less as well, and the heavy snowing has decreased significantly. The results of the analysis of satellite images and the NDSI index showed that the snow cover in Hamadan city has significantly decreased. This amount, compared to the area of the city, has decreased from about 20% of the snow cover to about 7% over the decade. As a result, water reserves have faced serious problems.



Keywords: Changes in the snowfall regime, NDSI, harmonic analysis, modeling, Hamadan